Staphylococcus aureus is a predominant cause of community-acquired and nosocomial infections. In addition, it is the most common cause of skin and skin structure infections, and endocarditis, and is the second most common cause of bacteremia. Despite the use of new generation antibiotics, morbidity and mortality associated with S. aureus infections remain unacceptably high. Persistent MRSA bacteremia (PB) represents an important subset of S. aureus infections, and correlates with particularly severe outcomes. Therefore, PB presents a significant therapeutic challenge to the medical community. Understanding the relevant molecular mechanisms of PB is essential to optimize therapy against life-threatening S. aureus infections. Our preliminary data indicate that PB clinical outcomes significantly correlated with differences in key pathogenic characteristics as compared with resolving MRSA bacteremia (RB). These preliminary data provide a solid foundation to investigate our central hypotheses: early agr activation is an important pathogenic signature in persistent MRSA bacteremia. To test our hypotheses, we will achieve the following integrated Specfic Aims: 1) Define agr RNAIII transcription, functionality and locus sequence profiles in vitro in an expanded collection of well- characterized PB vs. RB strains; and 2) Define agr transcription and functionality in vivo using the experiment IE model, and evaluate the impact of these agr profiles on innate MRSA virulence and antimicrobial efficacy outcomes in the model. Northern blot analyses, gene sequence, nucleic acid sequence-based amplification (NASBA) and quantitative RT-PCR will be employed in this project. These studies will significantly advance our understanding the pathogenesis of MRSA infections. Our long-term goal is to identify unique PB signatures for development of rapid diagnostic means and novel antimicrobial strategies against MRSA infections, such as PB.